High temperature superconductor element for a fault current limiter

Abstract

A high temperature superconductor fault current limiter and a method of using same. The fault current limiter comprises a high temperature superconductor material structure at least partially encapsulated in an epoxy having thermal conductivity properties that enable the superconductor to heat rapidly during a fault condition while preventing thermal runaway. The epoxy encapsulation decreases the critical current density of the superconductor material structure and increases the rate at which resistance increases with increasing current once the critical current density is exceeded. Preferably, the epoxy has thermal expansion properties approximately equal to the thermal expansion properties of the superconductor material structure.

We claim: 
     
       1. A high temperature superconductor fault current limiter for use in an electrical current carrying circuit capable of carrying a fault current, comprising: a high temperature superconductor material disposed in said electrical current carrying circuit, and   means for insulating said superconductor material for controlling heating of the high temperature superconductor material during a fault current excursion to increase electrical resistance and said insulating means further exhibiting a thermal conductivity to prevent thermal runaway.   
     
     
       2. The fault current limiter defined in claim 1, wherein said high temperature superconductor material comprises a filament having a diameter of less than two millimeters. 
     
     
       3. The fault current limiter defined in claim 1, wherein said means for insulating is selected from the group consisting of epoxy and filled thermoplastics. 
     
     
       4. The high temperature superconductor fault current limiter as defined in claim 1 wherein said high temperature superconductor material comprises a plurality of filaments electrically connected in parallel in the electrical current carrying circuit. 
     
     
       5. The fault current limiter as defined in claim 1, wherein said high temperature superconductor material comprises 123YBCO. 
     
     
       6. A method of producing a high temperature superconductor fault current limiter, comprising the steps of: providing a high temperature superconductor material having a critical current; and   substantially encapsulating said high temperature superconductor material in a thermally conductive epoxy having thermal expansion properties at least as great as thermal expansion properties of said high temperature superconductor material, thereby enabling said epoxy to shrink and to compress said high temperature superconductor material when cooled and further using said epoxy having a thermal conductivity and thickness such that a balanced quasi-stable thermal state is achieved during a fault condition while operating the superconductor material above the critical current.   
     
     
       7. The method as defined in claim 6, wherein said high temperature superconductor material comprises a filament having a diameter of less than two millimeters. 
     
     
       8. The method as defined in claim 6, wherein said epoxy comprises a layer of epoxy less than two millimeters thick. 
     
     
       9. The method as defined in claim 6, wherein said high temperature superconductor material comprises 123YBCO. 
     
     
       10. The method as defined in claim 6, wherein said high temperature superconductor material comprises a plurality of filaments electrically connected in parallel in the electrical current carrying circuit. 
     
     
       11. A method of producing a conductor for a high temperature superconductor fault current limiter, comprising the steps of: providing a high temperature superconductor material;   placing at least a portion of said high temperature superconductor material in a container; and   filling at least a portion of said container with epoxy to substantially encapsulate said high temperature superconductor material in a layer of epoxy of substantially uniform thickness said epoxy having a specific heat, thermal conductivity and a thickness such that electrical terminal resistance of the high temperature superconductor material increases in a controlled manner once its critical current density is exceeded, thereby preventing thermal runaway while operating the superconductor material above its critical current.   
     
     
       12. The method as defined in claim 11, wherein said high temperature superconductor material comprises a filament having a diameter of less than two millimeters. 
     
     
       13. The method as defined in claim 11, wherein said layer of epoxy is less than two millimeters thick. 
     
     
       14. The method as defined in claim 11, wherein said high temperature superconductor material comprises 123YBCO. 
     
     
       15. The method as defined in claim 11, wherein said high temperature superconductor material comprises a plurality of filaments electrically connected in parallel in the electrical current carrying circuit. 
     
     
       16. A method of controlling a fault current in an electrical current carrying circuit, comprising the steps of: providing a high temperature superconductor material of selected structural size and having a critical temperature;   coupling said high temperature superconductor material to the electrical current carrying circuit; and   substantially encapsulating said high temperature superconductor material in an epoxy layer having a coefficient of thermal expansion at least substantially equal to a coefficient of thermal expansion of said high temperature superconductor material, said epoxy layer further having thermal conductivity properties selected to allow heating of said high temperature superconductor material above its critical temperature to increase electrical resistance while avoiding thermal runaway by selecting a balance of parameters of the selected structural size of the high temperature superconductor material and the thermal conductivity of the epoxy layer.   
     
     
       17. The method as defined in claim 16, wherein said high temperature superconductor material comprises a filament having a diameter of less than two millimeters. 
     
     
       18. The method as defined in claim 16, wherein said epoxy layer is less than two millimeters thick. 
     
     
       19. The method as defined in claim 16, wherein said high temperature superconductor material comprises 123YBCO. 
     
     
       20. The method as defined in claim 16, wherein said high temperature superconductor material comprises a plurality of filaments individually encapsulated in epoxy and electrically connected in parallel in the electrical current carrying circuit.